bnxt_en: Fix tx push race condition.

Set the is_push flag in the software BD before the tx data is pushed to
the chip.  It is possible to get the tx interrupt as soon as the tx data
is pushed.  The tx handler will not handle the event properly if the
is_push flag is not set and it will crash.

Signed-off-by: Michael Chan <michael.chan@broadocm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>